1. Field of the Invention
This invention relates to an improved method and apparatus for transmitting digital data at high speeds via a parallel data bus, and more particularly, to a method and apparatus that provides a cost effective interface to a direct access storage device.
2. Description of the Prior Art
As will be appreciated by those skilled in the art, such factors as noise and loading limit the useful length of parallel busses operating at high data rates. In the prior art, the length of the bus must be taken into account in the system design and the bus length must be precisely as specified. Manufacturing tolerances associated with physical communication link (chips, cables, cord wiring, connectors, etc.) and temperature and variations in power supply voltage also limit the data rates on prior art busses comprised of parallel conductors. Further, many prior art computer systems transfer data synchronously with respect to a processor clock, so that a change in processor clock rate may require a redesign of the data transfer bus.
In a current large computer system, the I/O element can require more than 100 channel functions (as the middle stage/level in a hierarchically arranged busing network) between the highest level internal bus (fastest) and the more numerous and slower I/O controllers. In many system configurations, it is typical for up to 80% of the channels to be used for data transmission paths to the direct access storage device (DASD) data base via DASD I/O controllers. These numerous channels/paths can be required for connectivity to the data base, performance (access rate to the data base) or both.
In prior art systems, the channel's function is to execute, cooperatively with the attached I/O controller, channel programs (lists of channel/controller commands) which have been set up by application or operating system programs in host storage. Typical commands are to initiate the transfer of data between storage and the DASD attached to the I/O controller. Current channel functions are thus designed to transmit this data at a maximum rate (e.g., 18 MB/S) consistent with current DASD I/O controller capabilities.